The present invention concerns a tensioning device for tensioning a traction medium, especially a chain. Such a tensioning device can be used, for example, in the timing drive of internal combustion engines. The tensioning device can be attached to a third object, for example, to the motor block or cylinder head.
In the timing drive of machines such as, for example, internal combustion engines, a traction medium, for example, in the form of a chain or a belt, is typically used to synchronize two driven machine parts, for example, two cam shafts. For this purpose, it is necessary that the traction medium be constantly under tension in order to ensure satisfactory coupling. This is realized via a tensioning device exhibiting a supporting body element which it is attached to a machine element, for example, to the motor block, the cylinder head, or the timing housing cover. A known tensioning device such as that described, for example, in U.S. Pat. No. 2,963,819, further includes one or two spring-loaded, chain-engaging, and consequently chain-tensioning tracks movably supported on the body element via a guide bolt. Depending on the arrangement of the tensioning device, the traction medium is pressed by such a system either toward or away from the connecting line between the axes of rotation of the machine parts coupled by the traction medium. Previously known tensioning devices consist, however, of a number of individual parts which need to be produced and machined in elaborate fashion as individual parts and then assembled in a further manufacturing step. The elaborate and complicated manufacture and assembly of previously known tensioning devices disadvantageously affect production costs.
The present invention is therefore concerned with the problem of providing a tensioning device which can be produced simply and economically.
The problem with which the present invention is concerned is solved through the fact that in the case of a device for tensioning a traction medium, especially a chain or a belt, including a supporting body element attached to a third object and at least one tensioning track capable of being applied against the drawing means, the supporting body element and the tensioning track are joined together as one piece.
In the tensioning device according to the present invention, the essential elements of the apparatus, namely, the supporting body element and the one or (as can be further provided according to the invention) two tensioning tracks are thus arranged or joined together as one piece. The supporting body element and tensioning track(s) thus form a single one-piece part. As a result, the elaborate separate manufacture and assembly of individual parts such as occur in the state of the art are very advantageously eliminated. The tensioning device according to the present invention is therefore simply constructed and economically producible. Since the tensioning device according to the present invention can be made of plastic, it can be realized with special advantage in the form of a simple and convenient injection-molded part. The tensioning track can be provided, if necessary, with an additional antifriction coating.
According to a first embodiment of the present invention, the tensioning track and the supporting body element can be joined together via elastically deformable bar- or rib-like interconnecting sections. These elastically deformable interconnecting sections are designed to produce a high elastic force. This elastic property makes it possible to compensate for any tolerances in the timing drive or any changes in the drive resulting from heat expansion or wear. Similarly, operation-related positional changes in the tensioned chain, for example, chattering of the chain, can be absorbed or compensated in this way. The elastic property which makes it possible for the tensioning track to be movable with respect to the supporting body element can be realized through the fact that the interconnecting sections have an angled design. Alternatively, the interconnecting sections can be designed in the form of a honeycomb pattern.
In addition to the arrangement of elastically deformable interconnecting sections, one can, according to the present invention, position a spring element between the supporting body element and the tensioning track, through which the tensioning track is forced away from the supporting body element. The elastic force produced can be further increased and the tensioning properties further improved by this spring element. However, for the case in which the elastic interconnecting sections are designed to give sufficient elastic stiffness, the use of an additional spring element is not absolutely necessary.
If a plastic, preferably polyamide, is used as the material for the supporting body element and the tensioning track, further advantages result in connection with the described spring element: the temperature in the engine compartment of a motor vehicle can reach approximately 120xc2x0 C. The modulus of elasticity of the plastic is significantly lower at this elevated temperaturexe2x80x94in certain cases, down to a tenthxe2x80x94than at the lower temperature prevailing in the engine compartment with a cold motor. Under the tension load of the chain, the tensioning track (when no spring element is employed) could possibly be shifted in undesired fashion in the direction of the supporting body element, i.e., away from the chain. This could be disadvantageous especially when the motor is turned off and the tensioning track cools down and hardens in the shifted or lowered position since then the desired chain tension might possibly no longer be present. Under the elastic load of the spring element, this undesired shifting is checked or clearly reduced.
A further advantage the tensioning track formed entirely from plastic can be seen in the fact that the tensioning track is pressed against the chain by the elastic load over a large part of track""s extension in the running direction of the chain. In the case of conventional tensioning tracks of rigid design, the track is convexly curved on its side facing the chain. In the case of a newly installed chain which has not yet been stretched under load, the chain is so tautly tensioned that it runs almost simply tangent to the convexly curved tensioning track. The stretched chain, in contrast, hugs the tensioning track over a long run. In the case of the chain tensioner according to the present invention, the tensioning track and the spring can be designed and shaped for interplay with each other such that the convex curvature of the tensioning track on its side facing the chain can be changed by the elastic load and the force exercised by the chain, specifically such that both the newly installed chain as well as the stretched chain lie against a long run of the tensioning track.
According to another embodiment of the present invention, a spring element can be positioned or applied between the supporting body element and the tensioning track, with the supporting body element and the tensioning track being joined together via bar- or rib-like interconnecting sections such that the tensioning track can be deflected with respect to the supporting body element. In this case, the interconnecting sections are designed such that they exhibit only low elastic stiffness. In any case, they are designed and arranged such that the tensioning track can be deflected with respect to the supporting body element, which is realized via the spring element, to force the tensioning track away from the supporting body element. The spring element can be a coil spring positioned in an appropriately dimensioned recess between tensioning track and supporting body element. Alternatively, a leaf spring or leaf-spring packet can also be used as the spring element. For simple mounting of the spring element, retaining means for the same are provided on the supporting body element and/or in the area of the tensioning track. In the case of a coil spring this can be, for example, cylindrical projections on which the coil spring is pressed with its ends.
According to another embodiment of the present invention, the tensioning track and the supporting body element can be joined together by rigid bar- or rib-like interconnecting sections. In this embodiment, there are no elastic interconnecting sections and no spring element; the tensioning track can not be deflected as a whole. The supporting body element, interconnecting sections, and tensioning track(s) here, too, form a one-piece part whose tensioning action results from the fact that the presetting of the traction medium as well as the geometry and dimensioning of the tensioning device are adapted to each other such that the desired chain tension results upon installation of the tensioning device.
The tensioning device according to the present invention can be produced in addition as a metal construction as well as plastic, as previously described. This permits very simple low-cost production of each of the types of tensioning device described above. In these devices, the tensioning track can consist of a plastic exhibiting high abrasion resistance and the supporting body element, of a different plastic. Both plastics can be injected in a single operating step using a two-component injection process. In this way, it is possible to produce the tensioning track and supporting body element using different plastic materials which best meet the demands made on the tensioning track and the supporting body element. The supporting body element can further be reinforced with glass fibers in order to attain high basic strength. Polyamides, especially polyamide 66 or 46, have proven to be suitable plastics. In addition, they possess very good antifriction properties so that the traction medium can slide nicely over the track.
For simple attachment to a third object, for example, a motor block or the like, at least one mounting hole for receiving an attaching element, e.g., an attaching screw can be provided. Where preferred, two mounting holes can be provided. In the mounting hole itself, there can be positioned a metal sleeve via which the pressure forces of the attaching element, for example, the pressure forces of an attaching screw are accommodated. The metal sleeve can be pressed into the mounting hole or die-sunk therein, for which purpose ultrasonic die-sinking is useful. Alternatively, it is possible in the case of a plastic version of the tensioning device to spray the metal sleeve with the plastic used in forming the mounting hole.
As already described, two opposing tension tracks can be provided on the supporting body element, being joined as one piece thereto. Depending on the version involved, each tensioning track can be joined with the supporting body element via elastic or rigid interconnecting sections. This tensioning device thus possesses a taut run and a slack run each engaging different sections of the traction medium. Since higher forces naturally prevail along the taut run, it has proven advantageous to have a design in which the tensioning track and possibly the bar- or rib-like interconnecting sections joining the track to the supporting body element are stiffer here than along the opposing slack side. In addition, a spring element can also be located between the supporting body element and each of the tensioning tracks. In this case, one spring element can be stronger than the other, the stronger spring element being located in the more stiffly dimensioned taut run.